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20:39 <davidm123> is there a way to operate on only a subexpression?  for example, if I want to factor only the denominator of an expression.
20:39 <davidm123> the following factors the entire expression: factor( (1/(2*x) + 1/(2*y)) / (1/(2*z) + 1/(2*w)) );
20:39 <davidm123> I can use "part", but that eliminates the numerator: factor( part( (1/(2*x) + 1/(2*y)) / (1/(2*z) + 1/(2*w)), 2) );
20:43 <davidm123> I can preserve the numerator by reconstructing the expression %i1 as follows: part(%i1,1) / factor(part(%i1,2));
20:44 <davidm123> but in general, I'd prefer not to rewrite the algebra for reconstructing that.  That example with division is simple, but other expressions would be more complicated.
20:45 <davidm123> What I want to do is something like as following (using pseudo-code):  factor( (1/(2*x) + 1/(2*y)) / (1/(2*z) + 1/(2*w))  , part, 2 );
20:46 <davidm123> so that factor returns the full original expression but only operated on the given subexpression (the denominator)
20:46 <lindi-> hmm
20:47 <davidm123> sort of like what you do in Mathcad: select the subexpression and click "Factor".
20:48 <lindi-> i still haven't managed to get source level debugging work
21:38 <davidm123> well, this seems to be one approach...
21:38 <davidm123> matchdeclare(A,lfreeof([x]))$
21:38 <davidm123> defrule(r1,A,factor(A))$
21:39 <davidm123> apply1( (1/(2*x) + 1/(2*y)) / (1/(2*z) + 1/(2*w)), r1 );
21:39 <davidm123> or something like that.